1. Field of the Invention
The present invention relates to a hypereutectic white iron alloy that comprises chromium and nitrogen, as well as to articles such as pump components made therefrom (e.g., by sand casting).
2. Discussion of Background Information
High chromium white iron alloys find use as abrasion resistant materials for the manufacture of, for example, casings of industrial pumps, in particular pumps which come into contact with abrasive slurries of minerals. This alloy material has exceptional wear resistance and good toughness with its hypoeutectic and eutectic compositions. For example, high chromium white iron in accordance with the ASTM A532 Class III Type A contains from 23% to 30 wt. % of chromium and about 3.0% to 3.3 wt. % of carbon. However, in severely abrasive applications the wear resistance of these high chromium white iron alloys is not satisfactory due to a lack of a sufficient “Carbide Volume Fraction” (CVF). It is well known that increasing the content of both Cr and C can considerably improve the wear resistance of high chromium white iron alloys under severely abrasive conditions. For example, hypereutectic Fe—Cr—C alloys for hardfacing typically contain 4.5% C and 24% Cr. The amount of carbides and in particular, the CVF can be estimated from the following experimentally developed equation: CVF=12.33×% C+0.55× (% Cr+% M)−15.2% (M representing one or more carbide forming elements in addition to chromium, if any). For the above hardfacing alloy, CVF=(12.33×4.5%)+(0.55×24%)−15.2%=53.5%.
Hardfacing has the benefit of making an article wear resistant by cladding, i.e., by depositing a layer of an alloy of wear resistant composition thereon. However, hardfacing methods have disadvantages, including a limited thickness of the cladding, distortion of the article to be cladded, and high costs of labor, cladding material and equipment. Moreover, the cladding usually is susceptible to developing defects such as spalling and cracking due to thermal stresses and contraction, and it shows constraints with respect to thermal hardening.
Further, making (slurry) pump components such as pump casings by common foundry methods from hypereutectic high chromium white iron alloys is virtually impossible due to high scrap and rejection rates. Pump casings are large and heavy and are not uniform in thickness. For example, cross-sections in some areas of a pump casing may be up to 10 inch and the wall thickness in at least some parts thereof may be 1 inch or even higher. In view thereof, it is virtually impossible for a casting to cool uniformly in a sand mold, which results in stress induced cracking during cooling.
In particular, during solidification in a sand mold, hypereutectic high chromium cast iron forms a primary phase by nucleation and growth processes. Large primary chromium carbides, up to several hundreds microns in length, crystallize in the thick sections of the casting where the cooling is slower than in the remainder of the casting. These large primary carbides lower the fracture toughness of a casting, wherefore the casting usually cracks during the manufacturing process or later during application in the work field.
For the foregoing reasons, hypereutectic high chromium white cast iron alloys have in the past not been suitable for the sand casting of large parts and there have been various attempts to address this problem.
The background section of WO 84/04760, the entire disclosure of which is incorporated by reference herein, which primarily relates to high chromium white cast iron alloys of both hypoeutectic and hypereutectic compositions, describes the many failed attempts to develop satisfactory hypereutectic white iron alloys for castings, which combine wear resistance with fracture toughness. This document also describes various attempts to develop hypoeutectic compositions, and draws on attempts in the art to develop suitable hardfacing alloys as providing possible solutions to the wear resistance vs fracture toughness dilemma. However, according to WO 84/04760 the cracking problem of cast compositions is in fact predominantly solved by forming them as cast composites—namely by creating a composite component comprising the preferred alloy metallurgically bonded to a substrate, thus assisting with avoiding the likelihood of cracking upon cooling of the cast alloy. WO 84/04760 seeks to overcome the disadvantages of low fracture toughness and cracking with hypereutectic castings having greater than 4.0 wt, % carbon by ensuring the formation in a composite casting of primary M7C3 carbides with mean cross-sectional dimensions no greater than 75 μm, and suggests a variety of mechanisms for doing so. Thus, WO 84/04760 aims to overcome the problem by forming composite components and limiting the size of the primary M7C3 carbides in the alloy itself.
U.S. Pat. No. 5,803,152, the entire disclosure of which is incorporated by reference herein, also seeks to refine the microstructure of in particular, thick section hypereutectic white iron castings, in order to maximize the nucleation of primary carbides, thereby enabling an increase not only in fracture toughness but also in wear resistance. This refinement is achieved by introducing a particulate material into a stream of molten metal as the metal is being poured for a casting operation. The particulate material is to extract heat from, and to undercool, the molten metal into the primary phase solidification range between the liquidus and solidus temperatures. This method has the limitation of a difficult to achieve even distribution of the additive, a particulate material, into a stream of molten metal as the metal is being poured for a casting operation. The particulate material consists mainly of chromium carbides which contain about 10% C and 90% Cr and is added to the stream of molten metal in amounts of up to 10%. This addition of carbides increases the carbon and chromium concentrations in the already hypereutectic base alloy iron and causes a shift and extension of the interval between liquidus temperature and solidus temperature.
Substituting nitrogen for carbon is known for the production of High Strength Low Alloy Steels (HSLAS). The HSLAS comprise about 0.15% C, 0.03% N and 0.15% V. In this case it was shown that for every added 0.01% of C the strength increases by 5.5 MPa after thermo-mechanical processing, while for every added 0.001% of N the corresponding increase is 6 MPa. It was found that vanadium and nitrogen first form pure VN nuclei, which subsequently grow at the expense of solute nitrogen. When nitrogen is exhausted, the solute carbon precipitates and progressively transforms the nitrides into carbonitrides V(CyN1-y) instead of into precipitates of VC. These carbonitrides are of submicron size and crystallize in the face-centered cubic NaCl type crystal structure.
Another advantage of the substitution of nitrogen for carbon in iron alloys is described in U.S. Pat. No. 6,761,777, the entire disclosure of which is incorporated by reference herein. This patent discloses alloys containing from 0.01% to 0.7% of N and showing improved mechanical properties, in particular corrosion and wear resistance, due to nitrogen giving rise to the formation of carbonitride precipitates and solid solution strengthening.
Further, titanium nitride is produced intentionally within some steels by addition of titanium to an alloy. TiN forms at very high temperatures and nucleates directly from the melt in secondary steelmaking Titanium nitride has the lowest solubility product of any metal nitride or carbide in austenite, a useful attribute in microalloyed steel formulas.